1. Field of the Invention
The present invention relates to a ball joint and a ball joint production method.
2. Description of the Related Art
JP-A-2003-262213, for example, discloses a ball joint which includes a metal ball stud having a round head provided at one end thereof, a synthetic resin sheet covering an outer surface of the round head, and a bottomed cylindrical housing accommodating the round head and the resin sheet. The ball stud is pivotal about the round head relative to the housing. The round head is slidable on an inner surface of the resin sheet, as the ball stud is pivoted. The inside of the housing is filled with grease (particularly, the grease is provided between the resin sheet and the round head).
In production of the ball joint, an open end portion of the housing (an end portion of the housing to be formed with an opening) is swaged inward so as to be reduced in diameter as compared with the other portion of the housing. Thus, the round head of the ball joint is prevented from being withdrawn from the housing, and the resin sheet is pressurized. During the swaging of the open end portion of the housing, the round head and the resin sheet are generally retained in the housing.
If lubrication between the outer surface of the round head and a contact surface (inner surface) of the resin sheet is impaired in the ball joint, the round head and the resin sheet are rubbed against each other, whereby the resin sheet is abraded. This may reduce the service life of the ball joint. Therefore, the lubrication between the round head and the resin sheet should be properly maintained.
The inventors of the present invention contemplate to provide (distribute) minute grease retainers defined between the contact surface of the resin sheet and the outer surface of the round head over the entire contact surface to provide a sufficient amount of grease between the round head and the resin sheet. A conceivable approach to this is to preliminarily provide a multiplicity of minute groves in the entire contact surface of the resin sheet for retaining the grease.
In the production of the ball joint, however, the resin sheet is pressed against the round head to be deformed, when the open end portion of the housing is swaged. Therefore, the minute grooves provided in the contact surface of the resin sheet are collapsed. Thus, the aforementioned approach fails to provide the grease retainers between the outer surface of the round head and the contact surface of the resin sheet.